1. Field of Invention
The present invention relates to a fluorescent lamp with external electrodes and a backlight luminaire, and more particularly, to an improved backlight including an external fluorescent lamp in which the external electrodes are installed at both ends of the fluorescent lamp, and a method for driving the backlight.
2. Background of Invention
In general, a flat panel display is categorized into two types: an active lighting type and a passive lighting type. The active lighting type includes a flat panel cathode-ray tube, a plasma display panel, an electronic active lighting element, a fluorescent display, an active lighting diode, etc., where as the passive lighting type includes a liquid crystal display.
In the liquid crystal display, an image is formed not by a self-illumination provided by the incident light from the outside of the liquid crystal panel. To accomplish this, a backlight luminaire is typically installed at the rear end of the liquid crystal panel to enable the illumination. Hence, the image formed on the liquid crystal display can be seen even in dark places. It is highly desirable to produce a thin, light weight, and low-cost backlight luminaire that has high luminance, high efficiency, uniform luminance, and longer operation life. Highly efficient and long lasting lamp is desirable for use in notebook PCs to reduce the electrical power consumption, whereas high luminant lamp is desirable for use in regular PC monitors and TVs.
Two widely used methods as a backlight luminaire are a cold cathode fluorescent lamp (CCFL) and a flat fluorescent lamp. The CCFLs can be categorized into two types: (i) an edge light arrangement utilizing a plastic light guide, and (ii) a direct light arrangement in which repeated light sources are disposed on a plane in accordance with the arrangement of the light source with respect to the display face.
The above CCFL operates at a high luminance of about 30,000 cd/m2, and as a result has a shorter lamp life span. In particular, the edge light type is not suitable for a large screen panel as the luminance of the panel is weak even though the CCFL itself is highly luminescent. In the direct light type, it is impossible to connect the CCFLs in parallel arrangement and drive the backlight using a conventional inverter as the distance between the CCFLs has to be provided within a limited screen space to achieve desired illumination.
Meanwhile, the conventional flat-fluorescent lamp requires sufficient thickness to prevent the substrate made of glass from being damaged as the pressure between the upper and lower substrates is lower than the atmospheric pressure. As a result, the weight of the lamp tend to increase. In addition, in the conventional flat-fluorescent lamp, partitions and spacers in the form of a bead or cross are typically interposed between the upper and lower substrates in order to enlarge a screen area; thus, a uniform luminance cannot be achieved as the striped patterns of the partitions appear on the screen.
Accordingly, there is a need to develop a backlight source that is capable of ensuring high luminance and efficiency when placed in the back of a liquid crystal display.
Currently, there are various external electrode fluorescent lamp (EEFL) that are available as shown in FIG. 11. Although the EEFL tends to have a longer operation life than the CCFL, it has not been widely accepted as a backlight source due to the EMI and low efficiency. Moreover, the EEFL requires a larger power source using a high frequency of about several MHz. Furthermore, the EEFL has not been employed as a backlight source as its luminance and efficiency tend to be low as the LC-resonance type inverter designed for driving the CCFL is used for driving the EEFL.
FIG. 11 shows different types of the conventional external electrode fluorescent lamps. In particular, FIG. 11(a) illustrates a belt type external electrode with a pairs of the belt type electrodes installed on the cylinder of the glass tube driven typically at a high frequency of several MHz. The belt type EEFL (a) has an advantage in that additional electrodes can be installed even at an intermediate portion of the glass tube. This type of external electrode fluorescent lamps can attain a high luminance of several 10,000 cd/m2 by driving the lamps at a high frequency of several MHz. Moreover, the installation of the belt type electrodes in the intermediate portion of the glass tube is helpful to operate even at a higher frequency. However, there are some drawbacks in that a uniform and thin panel cannot be realized due to a decrease in the luminance of the electrode portion. In addition, the high frequency driving causes the undesirable EMI to be emitted, thus the efficiency of the electrodes becomes low. Furthermore, the high frequency power source is undesirable in designing a compact device using such a power source.
FIG. 11(b) illustrates a conventional external electrode in which metal capsules are bonded at the ends of the glass tube, and ferrodielectrics are applied to the inside of the metal capsules. This type of electrode is disclosed in U.S. Pat. No. 2,624,858 (Jun. 6, 1953) and typically employed to prevent the electric capacitive voltage drop caused by the thickness of the glass tube. However, the bonded portions of the electrodes can be easily damaged since coefficient of the thermal expansion of the glass tubes is different from that of the metal. However, if a fine glass tube, i.e., a cold cathode-ray tube, is used as the backlight source with an outer diameter of 2.6 mm and thickness of 0.5 mm or less, the metal capsules bonded to the glass tubes, as shown in FIG. 11(b), does not have to be used since the electric capacitive voltage drop due to the thickness of the glass tube is small.
FIGS. 11(c) and (d) illustrate lamps where the spaces at both ends of the glass tube are larger for achieving high luminance and efficiency. This type of external electrode is disclosed in U.S. Pat. Nos. 1,612,387 (Nov. 28, 1926) and 1,676,790 (Jul. 10, 1928). When the spaces at both ends of the glass tube are configured as shown in FIG. 11, the luminance and efficiency of the lamp increase. However, it is difficult to apply this type of structure to manufacture a fine tube to be used in a compact device.
FIG. 12 is a prior art circuit diagram showing an IC for driving the CCFL for use in the LCD panel. The circuit includes a lamp driving IC 100 having a plurality of I/O pins, a main electrical power circuit portion 120 having a half bridge circuit, and a lamp 140. The lamp driving IC 100 comprises a first pin 1 connected to an input voltage terminal; a second pin 2 connected to a predetermined minimum frequency terminal; a third pin 3 connected to a predetermined maximum frequency terminal; a fourth pin 4 connected to a ground voltage terminal; a fifth pin 5 connected to a feedback ground terminal; a sixth pin 6 connected to a predetermined comparative terminal; a seventh pin 7 connected to a predetermined internal high voltage terminal; and, a eighth pin 8 connected to a predetermined external control signal terminal for determining ON/OFF of the IC circuit. The main electrical power circuit portion 120 comprises a half bridge circuit which responds to the output signal of the predetermined pin of the lamp driving IC 100 and includes a plurality of passive elements. The lamp 140 is driven in response to a predetermined output signal of the main electrical power circuit portion 120.
As shown in FIG. 12, the power is supplied to the CCFL employed in the LCD backlight by a means of an inverter. The function of an inverter is to obtain a high voltage required for initiation and maintenance of the CCFL discharge from a low alternating voltage of several ten kHz obtained from the LC-resonance type inverter by a boosting transformer. Here, the waveform outputted from the inverter takes the shape of sine wave. This LC-resonance type inverter is helpful in designing a simple and highly efficient device. On the other hand, it is impossible to connect the CCFLs in parallel arrangement and drive the CCFLs using a single inverter. To this end, the backlight in the form of a direct light or plastic light guide is combined with the CCFLs, but requires the number of the inverter to correspond to the number of the CCFLs.
The direct light backlight, in which a plurality of the external electrode fluorescent lamps are disposed at the edge areas or on a plane of the plastic light guide, can be driven using a single inverter by connecting the EEFLs in parallel. The reason is that real current does not flow to the electrode as the electrode for the EEFL is not exposed at the discharge space. Thus, the wall charges are collected on both electrode portions. The discharge at both ends of the lamp is interrupted by the formation of a reverse voltage due to the wall charges. Then, another lamp is discharged, and likewise other wall charges are formed and discharged thereafter. Hence, a plurality of lamps can emit light using a single inverter. However, the sine waves generated by an inverter and used for driving the CCFLs cannot efficiently control the wall charges; thus, it produces much lower luminance and efficiency than that of the EEFL having a single tube. In addition, when a plurality of EEFLs interconnected in parallel are driven by a single inverter, the number of active lighting EEFLs is limited as the time period to which a high voltage is applied during one cycle is limited. Therefore, a uniform luminance cannot be realized when a number of the EEFLs are disposed in a plane as a backlight source.
As mentioned above, even if the EEFLs can be driven by the LC-resonance type inverter of several ten kHz to drive the CCFL, the backlight consists of the EEFL cannot be efficiently realized. Furthermore, when adopting the conventional high frequency of the EEFLs at several MHz, the problems of EMI, low efficiency and miniaturization of power source, etc. cannot be easily overcome.
The present invention relates to a backlight source including external electrode-type fluorescent lamps capable of being driven in a parallel connection, wherein the external electrodes formed at electrodeless glass tubes on the outer portions of a plastic light guide.
The present invention provides external electrode fluorescent lamps capable of obtaining high luminance and efficiency using a low frequency of 100 kHz or lower.
The present invention provides partition-type fluorescent lamps used as a backlight source, wherein the plurality of fluorescent lamps with external electrodes are disposed between the upper and lower substrates and used as partitions. The present invention is contemplated to solve the problems of driving the backlight employing the fluorescent lamps mentioned above and driving the backlight made by arranging these lamps in a plane orientation.
According to one aspect of the present invention, the inventive external electrode fluorescent lamp comprises a glass tube into which a discharge gas is injected, wherein the inner peripheral wall is coated with a layer of fluorescent substance and both ends of the tubes are then hermetically sealed; and, end-cap type external electrodes configured to have an L-shape, a C-shape, a helical shape or a wave shape to wrap both ends of the glass tube.
According to another aspect of the present invention, the inventive backlight source includes a plastic light guide; fluorescent lamps disposed at the edges of the plastic light guide and includes glass tubes into which a discharge gas is injected and the inner peripheral walls are coated with a layer of fluorescent substance, wherein both ends of the glass tubes are hermetically sealed; end-cap type external electrodes for wrapping both ends of the glass tubes; and, a switching inverter connected to the external electrodes for applying square wave signals with a frequency of 100 kHz or lower to the external electrodes. The external electrode fluorescent lamps include a plurality of external electrode fluorescent lamps interconnected in parallel.
According to another aspect of the present invention, the inventive backlight source includes a plurality of external electrode fluorescent lamps interconnected in parallel and includes glass tubes into which a discharge gas is injected, wherein the inner peripheral walls are coated with a layer of fluorescent substance and both ends of the glass tubes are then hermetically sealed; end-cap type external electrodes for wrapping both ends of the glass tubes; electrode connecting lines for connecting the end-cap type external electrodes of the plurality of external electrode fluorescent lamps in parallel; a reflecting plate; a diffusing plate; and, a switching inverter connected to the electrode connecting lines for applying square wave signals with a frequency of 100 kHz or lower to the electrode connecting lines. The reflecting plate further includes a plurality of triangular stands interposed between the external-electrode fluorescent lamps. The reflecting plate is shaped in wave form for wrapping the external electrode fluorescent lamps. The backlight further includes a plastic light guide having diffusing grooves in which the external electrode fluorescent lamps are seated. The reflecting plate is in the form of triangular sawteeth, and the external electrode fluorescent lamps are disposed along the triangular sawteeth.
According to another aspect of the present invention, the inventive backlight source includes a plurality of glass tubes into which a discharge gas is injected, wherein the inner peripheral walls are coated with a layer of fluorescent substance and both ends of the glass tubes are then hermetically sealed; socket-type multiple capsule electrode structures having a plurality of parallel-connected external electrode with which the glass tubes are coupled; a reflecting plate; a diffusing plate; and, a switching inverter connected to the socket-type multiple capsule electrode structures for applying square wave signals with a frequency of 100 kHz or lower to the socket-type multiple capsule electrode.
According to another aspect of the present invention, the inventive backlight source includes external electrode fluorescent lamps with external electrode portions thereof alternately disposed and transversely overlapped with each other in the middle of a panel; a reflecting plate; a diffusing plate; and, a switching inverter connected to the external electrodes for applying square wave signals with a frequency of 100 kHz or lower to the external electrodes. Each of the fluorescent lamps includes a glass tube into which a discharge gas is injected, wherein the inner peripheral wall is coated with a layer of fluorescent substance and both ends of the glass tube are hermetically sealed; and, capsule type external electrodes for wrapping both ends of the glass tube. The external electrodes of the external electrode fluorescent lamps are made of conductive transparent electrode materials.
According to another aspect of the present invention, the inventive backlight source includes an upper substrate with an upper layer of fluorescent substance applied on a bottom surface of the upper substrate; a lower substrate with a lower layer of fluorescent substance applied on a top surface of the lower substrate and installed to be opposite end of the upper substrate; edge supporting stands interposed between the upper and lower substrates for hermetically sealing the upper and lower substrates; external electrode fluorescent lamps installed at a predetermined interval above the lower substrate; electrodes formed at the corresponding outer surfaces on both sides of the assembled upper and lower substrates, respectively, and connected to the electrode connecting lines to which an alternating current type power source is applied; a switching inverter connected to the electrodes for applying square wave signals with a frequency of 100 kHz or lower to the electrodes; and, a discharge gas injected into an inner space upon sealing the upper and lower substrates. Each of the fluorescent lamps includes a glass tube into which a discharge gas is injected, wherein the inner peripheral wall is coated with a layer of fluorescent substance and both ends of the glass tube are hermetically sealed; and, capsule type external electrodes for wrapping both ends of the glass tube. The external electrode fluorescent lamps are not connected to the electrodes but disposed within the upper and lower substrates in a floating state.
According to further aspect of the present invention, the inventive backlight source includes an upper substrate with an upper layer of fluorescent substance applied on a bottom surface of the upper substrate; a lower substrate with a lower layer of fluorescent substance applied on a top surface of the lower substrate and installed to be opposite to the upper substrate; edge supporting stands interposed between the upper and lower substrates for hermetically sealing the upper and lower substrates; multiple capsule type electrode structures constructed by coupling upper and lower electrodes having surfaces coated with ferrodielectrics and grooves at a predetermined interval and then installed respectively on the inner portions at both ends of the lower substrate; glass tubes arranged in parallel coupled with, the grooves of the multiple capsule type electrode structures installed respectively on the inner portions at both ends of the lower substrate; electrode connecting lines connected to the multiple-capsule type electrode structures; a switching inverter connected to the electrode connecting lines for applying square wave signals with a frequency of 100 kHz or lower to the electrode connecting lines; and, a discharge gas injected into an inner space upon sealing the upper and lower substrates. Each of the glass tubes has a discharge gas injected therein and an inner peripheral wall coated with a layer of fluorescent substance. Both ends of each of the glass tube are then hermetically sealed.
According to further aspect of the present invention, the inventive switching inverter constitutes a bridge circuit by four FETs A, B, C and D. A DC is applied to the drains of the FETs A and C; sources of the FETs B and C are grounded; sources of FETs A and C are connected to the drains of the FETs B and D, respectively; and, a boosting transformer is connected between a connection point of the FETs A and B and a connection point of the FETs C and D. A square wave outputted from the switching inverter includes an overshooting.
According to further aspect of the present invention, the inventive driving method for driving a backlight with a plurality of external electrode fluorescent lamps interconnected in parallel comprises the steps of: dividing the plurality of external electrode fluorescent lamps into a plurality of predetermined regions; connecting identical electrode connecting lines to external electrodes of the fluorescent lamps in the respective divided regions, respectively; connecting switching inverters for outputting square waves to the electrode connecting lines connected to the respective divided regions, respectively; applying an identical gate signal to each of the switching inverters; and, supplying the electrode connecting lines with the in-phase square waves from the switching inverters in response to the gate signal. The switching inverter constitutes a bridge circuit by four FETs A, B, C and D. A DC is applied to the drains of the FETs A and C, sources of the FETs B and C are grounded, sources of the FETs A and C are connected to the drains of the FETs B and D, respectively, and a boosting transformer is connected between a connection point of the FETs A and B and a connection point of the FETs C and D.